Technological development and increased demand for mobile devices have led to rapid increase in the demand for secondary batteries as energy sources. Among such secondary batteries, lithium secondary batteries having high energy density, high operating voltage, long cycle span and low self-discharge rate are commercially available and widely used.
In addition, increased interest in environmental issues has recently brought about a great deal of research associated with electric vehicles (EV) and hybrid electric vehicles (HEV) as alternatives to vehicles using fossil fuels such as gasoline vehicles and diesel vehicles which are main causes of air pollution. Such electric vehicles generally use nickel-metal hydride (Ni-MH) secondary batteries as power sources. However, a great deal of study associated with use of lithium secondary batteries with high energy density, discharge voltage, and output stability is currently underway and some are commercially available.
A lithium secondary battery has a structure in which a non-aqueous electrolyte containing a lithium salt is impregnated into an electrode assembly comprising a cathode and an anode, each including an active material coated on a current collector, with a porous separator interposed between the cathode and the anode.
Lithium cobalt-based oxide, lithium manganese-based oxide, lithium nickel-based oxide, lithium composite oxide and the like are generally used as cathode active materials of lithium secondary batteries. Carbon-based materials are generally used as anode active materials. Use of silicon compounds, sulfur compounds and the like as anode active materials is also under consideration.
However, lithium secondary batteries have various problems, some of which are associated with fabrication and operating properties of an anode.
First, regarding anode fabrication, a carbon-based material used as an anode active material is highly hydrophobic and thus has low miscibility with a hydrophilic solvent, thereby reducing dispersion uniformity of solid components, in the process of preparing a slurry for electrode fabrication. In addition, hydrophobicity of the anode active material complicates impregnation of highly polar electrolytes in the battery fabrication process. Thus, electrolyte impregnation is a bottleneck in the battery fabrication process, greatly decreasing productivity.
Addition of a surfactant as an additive to an anode, an electrolyte or the like has been suggested as a possible solution to the problems. However, surfactants are unsuitable due to side effects upon operating properties of batteries.
On the other hand, regarding the operating properties of an anode, the carbon-based anode active material induces an initial irreversible reaction since a solid electrolyte interface (SEI) layer is formed on the surface of the carbon-based anode active material during an initial charge/discharge (activation) cycle. Removal (breakage) and reformation of the SEI layer through repeated charge/discharge cycles also causes depletion of the electrolyte, thereby reducing battery capacity.
Various methods, such as formation of an SEI layer with much stronger bonding to the anode active material and formation of an oxide layer or the like on the surface of the anode active material, have been attempted to solve these problems. However, these methods are unsuitable for commercialization due to problems such as deterioration in electrical conductivity caused by the oxide layer and deterioration in productivity caused by additional processes.
In addition, it is difficult to form an oxide layer with different properties on a highly non-polar anode active material and thus forming a uniform oxide layer inherently increases process cost.
Thus, there is a great need for secondary batteries capable of fundamentally solving these problems.